The present invention relates to flat plate optical components including beam splitters, partial polarizers and polarization separators, and, in addition, to an optical system for optical disk readers which incorporates such flat plate components.
In magneto optics recording systems which are based upon the Faraday rotation effect, traces are produced on the magneto optics medium so that the Faraday rotation effect varies along the trace according to the signal that is recorded on the track. The present invention relates to a system for reading such traces which uses flat plate optical components.
FIG. 1 schematically depicts an optical system 10 for reading such traces. A linearly polarized laser beam 11 is focused to a small spot on the track of a target disk 12. Specifically, linearly polarized light beam 11 from a laser source 13 is collimated by lens 14 and then traverses beam splitter 16 to an objective lens 17 which focuses the resulting light beam 21 into a small spot 18 on the disk 12. Typically, reader system 10 will also include a conventional system (not shown) for scanning the beam 21 across the disk 12.
The incident beam 21 is reflected from the disk 12 back through the focusing lens 17. The reflected beam 22 is still linearly polarized, but the axis of polarization may have been shited slightly by any Faraday rotation effect of the magneto optics medium coated on the disk 12. The shift of the axis will be small, that is, in the order of 0.1.degree. to 1.degree.. The signal recorded on the track is detected by measuring this shift in the polarization axis of the reflected beam 22 as the focused spot is scanned along the trace. One way to sense the change of the axis is to use a polarization separator 24 to resolve the output beam 23 from the beam splitter 16 into two orthogonally linearly polarized component beams P and S, each polarized at 45.degree. to the original axis of polarization.
Referring to FIG. 2, the intensity 28 and 29 of the two beams S and P will be exactly equal when there is no Faraday rotation (i.e., P=S). Referring also to FIG. 1, the resulting beams P and S can be directed by a collecting lens 26 to impinge on the electronic optical-intensity detectors of assembly 27. Conventionally, a circuit (not shown) is provided for electronically subtracting the signals from the two detectors of the detector assembly 27. In this case the circuit will provide essentially a zero output signal (P-S=0).
As indicated schematically in FIG. 3, rotation of the polarization axis through a small angle, .alpha., as the result of the Faraday rotation effect at disk 12, FIG. 1, will increase the intensity in one of the beams over that of the other (P&gt;S). Subtraction of the output from the two detectors of the assembly 27 will then result in a positive signal, (P-S&gt;0).
The effect of the rotation of the polarization axis can be enhanced by placing a partial polarizer in the returning beam 22 that favors the axis of polarization (S) that is perpendicular to the original axis (P).
Referring to FIG. 4, the division of the beam 22 into the two orthogonally polarized components can be done conventionally using a Wollaston prism 31 as the polarization separator 24 shown in FIG. 1. The Wollaston prism 31 consists of two pieces 32 and 33 of birefringent crystalline material such as calcite or quartz. The two pieces 32 and 33 are cemented together along surface 34 with their optical axes perpendicular. As a result, the assembled prism 31 will deflect the beams, P and S, of mutually perpendicular polarization at slightly different angles, so that they can be separated by lens 26 for detection.
Prisms, such as Wollaston prisms, have several advantageous characteristics when used as polarization separators. For example, the Wollaston prism provides a straight through beam path and does not require that the beam be collimated. In addition, prisms provide a very high separation ratio and the same path length for the P and S polarization. However, in volume commercial applications prisms have several overriding disadvantages. First, the desired prism materials such as calcite are expensive and scarce. Second, the prism components are difficult and costly to fabricate and assemble. In addition, passive surfaces such as 36 and 37 require an anti-reflection coating or matching to other optical elements. Such coatings are difficult and costly, particularly since the small prisms must be coated individually.
Prisms frequently are used as the beam splitter 16 and can be provided with a partial polarizer coating. However, as mentioned above, the materials used in prisms are expensive, the passive surfaces must be anti-reflection coated or matched to the next optical element(s), and the prisms are difficult and costly to fabricate, coat and assemble.